In signal processing, it is often necessary to convert an analog signal to a digital representation which is proportional to the amplitude of the original analog signal and vice versa. This A/D conversion process is essential to any application in which a computer or other digital processor is controlling or monitoring processes, experiments, or whenever digital techniques are applied to traditionally "analog" jobs. For example, A/D converters are increasingly being used in a wide variety of applications ranging from audio and video processing to measurement instrumentation, medical diagnostics, etc. Furthermore, applications in which digital information is converted to an intermediate analog form for transmission or storage (e.g., various digital modems) require A/D converters to convert these intermediate analog signals back to a digital format. In short, A/D converters can be used in a host of different applications.
FIG. 1 shows a block diagram of a typical prior art A/D converter. An analog source 101 is filtered by an anti-aliasing filter 102 to ensure that the signal to be sampled, does not include significant energy above half the sample rate where such signals would otherwise produce in-band interference due to aliasing effects. Next, sampler 103 samples the filtered analog signal. In other words, sampler 103 discretizes the time scale. Provided that the sampling clock rate f.sub.s is more than twice the bandwidth of the analog signal (i.e., the Nyquist frequency), the original analog signal may be reconstructed without loss of information. Finally, the quantizer 104 converts the continuous amplitude sampled signal to a discrete set of values, which are output as a digital sequence.
Prior art A/D converters generally require a number of expensive and complex high-precision devices. For example, the anti-aliasing filter 102 would ideally have a fiat magnitude response over the entire desired:d signal band while attenuating those frequencies above the Nyquist frequency to a level below the systems dynamic range floor. Thus, a high performance anti-aliasing filter is required to obtain high resolution with minimal distortion. In addition, the sampler 103 must accurately acquire values and hold them with minimal drift, providing enough time for the converter to compare the sampled analog signal to a set of reference levels. Thus, a high performance n-bit converter requires a sampler with better than one part in 2.sup.n accuracy. Similarly the quantizer must determine which of 2.sup.n possible reference values is closest to the sampled input. This implies a set of reference signals with better than one part in 2.sup.n accuracy. In addition one or more comparators with offset errors below one part in 2.sup.n of full scale are required. To illustrate these challenges a 12-bit A/D converter with an input range of 2 V should accurately resolve 500 .mu.V differences in its input signal. Thus the total sampler acquisition and holding error, plus the error in reference values, plus the comparison error must be less then 500 .mu.V. It is rather difficult and costly to achieve this degree of accuracy.
In an effort to overcome these problems, oversampled A/D converters were developed. Sigma-delta oversampled A/D converters are an outgrowth and extension of delta modulation techniques. Delta modulators quantize the sample to sample change in the input signal, rather than the absolute value of that signal. FIG. 2A shows a block diagram of a typical prior art delta modulator and its corresponding waveforms. The analog signal is summed by summer 201 with a negative feedback signal from the integrator 202. Essentially, the integrator 202 tries to predict the input analog signal x(t). The current prediction error term, x(t)-x(t) is then quantized by a one-bit quantizer 203 to make the next prediction. FIG. 2B shows a block diagram of a typical delta demodulator and its corresponding output waveform. It can be seen that the received signal is integrated by integrator 204 and then smoothed by a low pass filter 205.
By taking advantage of delta modulation principles, sigma-delta A/D converters can utilize a low cost, low resolution one-bit quantizer. Furthermore, high resolution can be achieved by implementing a very high over-sampling rate (i.e., sampling at a rate tens of times faster than the Nyquist rate) and then down converting from that high sampling rate through a decimation process. In this way, the large quantization error of the one bit quantizer is spread over a wider frequency range thus reducing the error in the desired band. Moreover, various degrees of noise shaping can be implemented to further reduce the in-band error.
Although sigma-delta AID converters offer some advantages over conventional "Nyquist" A/D converters, sigma-delta converters, nonetheless, suffer from several drawbacks. One of these drawbacks pertains to the one-bit quantizer, which usually takes the form of a comparator. Inexpensive comparators typically have too much delay time, which could result in stability problems for the sigma-delta loop (since delay is equivalent to a phase shift which increases with increasing frequency). On the other hand, comparators fast enough to have a benign delay are prohibitively expensive.
Another disadvantage is flue to the fact that the integrator(s) are frequently fabricated using switched capacitor circuit techniques. These techniques typically require a mixed analog and digital integrated circuit (IC) process. Use of a mixed analog and digital IC process has several drawbacks as compared to the use of a pure digital process.
A primary disadvantage to the use of a mixed analog-digital IC process is that mixed signal IC processes and libraries typically lag digital IC processes and ASIC libraries by one generation or 12 to 16 months. Thus, at any point in time, an all digital implementation is lower cost by one process generation.
A second disadvantage of a mixed analog-digital IC process is that it takes significantly longer to bring an IC (or product employing that IC) to market. A primary reason that customized digital ICs can be fabricated more quickly is that IC vendors stock digital gate arrays which have many of the processing steps already applied. Thus, all that is required to complete the IC fabrication process is application of custom interconnections. Other reasons that digital IC fabrication is quicker than mixed fabrication include reduced layout time, reduced test generation and debug time and a much higher probability of first time success. In many cases, time to market is of critical importance to the success of a new product and in all these cases an IC architecture which utilizes all digital components has a significant advantage.
A third drawback to the requirement for a mixed analog-digital IC process is that digital IC process dimensions are continually being reduced in order to reduce production costs. It requires significantly less time and money to translate an all digital design to a smaller geolmetry process than it does to translate a mixed analog digital design. Thus, a product with an all digital IC architecture can be more readily cost reduced thereby extending its profitabire life.
A fourth disadvantage of mixed analog-digital IC architectures is the sensitivity of analog structures to temperature variation. For example, it is extremely difficult to make a switched capacitor integrator function well from -40.degree. C. to +85.degree. C. In summary a product architecture which maximizes digital gate content can be brought to market more quickly, provide for future cost reductions and enable wide temperature range functionality.
Thus, there is a need in the prior art for a high resolution sigma-delta A/D converter with an architecture having an all-digital core and minimal analog components. It would be preferable if the comparator and loop filter (e.g., integrator) of such an architecture could be implemented using the digital logic cells of a gate array. It would be of further advantage if these digital components could be employed in such a way that the converters performance was better than if digital gates were not used.